1. Field of the Invention
This invention relates to a method for determining a urea concentration in an aqueous solution and an apparatus therefor. This invention may be suitably applied to, for example, a method and an apparatus for determining a urea concentration in a condensate processed in a condensate processing facility during a urea production process, and an apparatus that is capable of sending a signal for controlling a destination of the condensate, based on a urea concentration to a controller such as a distributed control system (DCS).
2. Description of the Related Art
In the past, techniques such as colorimetry or application of a nitrogen analyzer have been used to determine a concentration of urea in a solution. After removing dissolved urea, ammonia and carbon dioxide from the condensed water, the condensed water is then recycled to the production process for being reused as water required in the production process. The term “condensate” as used herein refers to condensed water discharged from, for example, a urea production process, which contains, e.g., urea, ammonia and carbon dioxide.
For example, a method for determining a urea concentration in a solution by colorimetry has been disclosed in Japanese Laid-open Patent Publication No. 2000-338099. However, due to a flaw in the colorimetry measurement system, subtle turbidity in a sample may disturb correct determination of the urea concentration using colorimetric determination method.
A second method using a nitrogen analyzer measures the amount of nitrogen by combusting a measuring sample in the presence of a catalyst to convert nitrogen into nitrogen monoxide and determining a nitrogen monoxide concentration. One drawback of this method is that a nitrogen analyzer determines a total nitrogen content in the measuring sample, and cannot selectively determine a urea concentration.
Additionally, in both methods, a calibration curve must be created using a dedicated standard solution before measurement and precise dilution. The creation of the calibration curve requires additional work, and may introduce inaccuracies from the operator's calculations. Moreover, it is difficult to quickly determine only a urea concentration in a solution in which urea and ammonia coexist using the above-described techniques.
Japanese Laid-open Patent Publication S59-160746 (1984) describes that a urea concentration in a solution is determined utilizing an electric conductivity. This method utilizes an enzyme, particularly for determining a urea level in blood. According to this method, urea concentration is determined by immobilizing an enzyme capable of decomposing urea into ammonia and carbon dioxide on a membrane, passing a solution containing urea through the membrane, absorbing ammonia decomposed by the enzyme into an absorbent solution and measuring an electric conductivity of the absorbent solution. However, a decomposition state (a decomposition rate) for urea may vary depending on enzyme activity and may be susceptible to temperature changes at the place where the membrane is fixed. Much maintenance effort is, therefore, required for keeping higher measuring precision. Furthermore, due to membrane deterioration over time, the membrane must be replaced, leading to troublesome maintenance. In addition, this measuring procedure requires a separate buffer solution during measurement, leading to an increase in running costs.
Japanese Laid-open Patent Publication H06-184085 (1994) describes a process for controlling a urea production plant using electric conductivity. This process involves measuring an ammonia concentration in an ammonium carbamate using an electric conductivity and controlling the total amount of water supplied to a urea synthesizing tube, but does not involve determination of a urea concentration. As a further problem, it requires precise dilution and expensive reagents and catalyst, leading to a higher cost for maintenance.
For example, a processed condensate from a condensate processing facility in the urea production process contains urea. Thus, when the condensate is effectively utilized for, e.g., boiler feed water, quality determination for the fed condensate is time consuming. It is therefore difficult to quickly determine whether the condensate meets the conditions for boiler feed water. An elevated urea concentration in the condensate may result in hydrolysis of urea into ammonia and carbon dioxide in a boiler, which may then cause internal corrosion of the boiler, leading to difficulty in effective utilization of the condensate and thus to difficulty in the conservation of energy and resources.
In light of the above-described difficulties, an objective of the present invention is to provide a method and an apparatus whereby urea concentration in a solution containing urea may be quickly and accurately determined using a convenient and inexpensive apparatus, without dilution of a solution to be measured and without creating a calibration curve upon measurement. Another objective of this invention is to provide a method and an apparatus for determining a urea concentration, whereby on-line water quality management of a urea-containing solution may be continuously conducted.